PROTEIN-KINASES AS MEDIATORS OF FLUID SHEAR-STRESS STIMULATED SIGNAL-TRANSDUCTION IN ENDOTHELIAL-CELLS - A HYPOTHESIS FOR CALCIUM-DEPENDENTAND CALCIUM-INDEPENDENT EVENTS ACTIVATED BY FLOW

Fluid shear stress regulates endothelial cell function, but the signal transduction mechanisms involved in mechanotransduction remain unclea r. Recent findings demonstrate that several intracellular kinases are activated by mechanical forces. In particular, members of the mitogen- activated protein (MAP) kinase family are stimulated by hyperosmolarit y, stretch, and stress such as heat shock. We propose a model for mech anotransduction in endothelial cells involving calcium-dependent and c alcium-independent protein kinase pathways. The calcium-dependent path way involves activation of phospholipase C, hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PIP2), increases in intracellular calcium a nd stimulation of kinases such as calcium-calmodulin and C kinases (PK C). The calcium-independent pathway involves activation of a small GTP -binding protein and stimulation of calcium-independent PKC and MAP ki nases. The calcium-dependent pathway mediates the rapid, transient res ponse to fluid shear stress including activation of nitric oxide synth ase (NOS) and ion transport. In contrast, the calcium-independent path way mediates a slower response including the sustained activation of N OS and changes in cell morphology and gene expression. We propose that focal adhesion complexes link the calcium-dependent and calcium-indep endent pathways by regulating activity of phosphatidylinositol 4-phosp hate (PIP) 5-kinase (which regulates PIP2 levels) and p125 focal adhes ion kinase (FAK, which phosphorylates paxillin and interacts with cyto skeletal proteins). This model predicts that dynamic interactions betw een integrin molecules present in focal adhesion complexes and membran e events involved in mechanotransduction will be integrated by calcium -dependent and calcium-independent kinases to generate intracellular s ignals involved in the endothelial cell response to flow.